U.S. patent number 5,359,760 [Application Number 08/046,965] was granted by the patent office on 1994-11-01 for method of manufacture of multiple-element piezoelectric transducer.
This patent grant is currently assigned to The Curators of the University of Missouri on behalf of the University. Invention is credited to Lawrence J. Busse, Wayne Huebner, Jeffry W. Stevenson.
United States Patent |
5,359,760 |
Busse , et al. |
November 1, 1994 |
Method of manufacture of multiple-element piezoelectric
transducer
Abstract
An improved method for fabrication of a multiple-element
piezoelectric transducer and the transducer produced thereby. A
green precursor tape is produced by doctor-blade tape-casting of a
slurry containing lead zirconate-titanate (PZT) powder. After
drying, individual strips of the tape are stacked between flat
plates of previously sintered PZT, and sintered to form PZT strips;
Pb from the previously sintered PZT plates makes up any Pb lost
from the surfaces of the tape strips during sintering. The PZT
strips are stacked interposed by layers of a thermoplastic polymer,
and heated to a temperature above the melting point of the polymer,
forming a laminate block. This block is then sliced perpendicular
to the plane of the layers, forming slabs of alternate PZT and
polymer layers; the slabs are then sliced perpendicular to the
first slicing planes, forming strips of alternating PZT and polymer
material. Electrodes are then added to complete the transducer
assembly.
Inventors: |
Busse; Lawrence J. (Littleton,
CO), Stevenson; Jeffry W. (Rolla, MO), Huebner; Wayne
(Rolla, MO) |
Assignee: |
The Curators of the University of
Missouri on behalf of the University (Columbia, MO)
|
Family
ID: |
21946325 |
Appl.
No.: |
08/046,965 |
Filed: |
April 16, 1993 |
Current U.S.
Class: |
29/25.35;
310/358; 310/800 |
Current CPC
Class: |
B06B
1/0622 (20130101); H01L 41/37 (20130101); A61B
8/4483 (20130101); Y10S 310/80 (20130101); Y10T
29/42 (20150115) |
Current International
Class: |
B06B
1/06 (20060101); H01L 41/22 (20060101); H04R
017/00 () |
Field of
Search: |
;29/25.35
;310/334-337,357,358,800 ;264/63 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
A Precision Tape Casting Machine for Fabricating Thin Ceramic
Tapes, R. B. Runk and M. J. Andrejco, Western Electric Co.,
Engineering Research Center, Princeton, N.J., Ceramic Bulletin,
vol. 54, No. 2 (1975). .
Tape Casting: The Basic Process for Meeting the Needs of the
Electronics Industry, Richard E. Mistler, Keramos Industries, Inc.,
Morrisville, Pa. 19067, Ceramic Bulletin, vol. 69, No. 6, 1990.
.
Tape Casting, Richard E. Mistler, Keramos Industries, Inc.,
reprinted from Engineered Materials Handbook, The Materials
Information Society, vol. 4: Ceramics and Glasses. .
Processing Parameters and Electric Properties of Doctor-Bladed
Ferroelectric Ceramics, Chandler Wentworth and George W. Taylor,
RCA Laboratories, Princeton, N.J., Ceramic Bulletin, vol. 46, No.
12 (1967)..
|
Primary Examiner: Hall; Carl E.
Attorney, Agent or Firm: de Angeli; Michael M.
Government Interests
GOVERNMENT INTEREST
This invention was made with government support under Contract No.
HL44230 awarded by the National Institutes of Health. The
government has certain rights in the invention.
Claims
What is claimed is:
1. A method for manufacture of a plurality of flat plates of lead
zirconate-titanate (PZT) piezoelectric material, comprising the
steps of:
preparing a slurry comprising PZT powder in a liquid binder;
forming a green precursor tape by tape-casting said slurry;
allowing said green precursor tape to dry;
cutting said green precursor tape into strips;
stacking said strips of green precursor tape, said strips being
interspersed by and in direct contact with flat spacer members
comprising sintered PZT;
sintering said stacked strips of precursor tape and PZT spacer
members, forming flat PZT plate members; and
removing said spacer members from said flat PZT plate members.
2. The method of claim 1, wherein said sintering step is performed
in a furnace, and a quantity of Pb-containing material is placed in
said furnace during said sintering step.
3. The method of claim 2, wherein said Pb-containing material is
PbZrO.sub.3.
4. The method of claim 2, wherein the temperature within said
furnace during said sintering step is gradually increased from room
temperature to approximately 500.degree. C., followed by
maintenance of the temperature within said furnace at approximately
1250.degree. C. for at least on the order of 0.5 hours.
5. A method for manufacture of a multiple-element piezoelectric
transducer, comprising the steps of:
preparing a slurry comprising PZT powder in a liquid binder;
forming a green precursor tape by tape-casting said slurry;
allowing said green precursor tape to dry;
cutting said green precursor tape into strips;
stacking said strips of green precursor tape interspersed with flat
spacer members comprising sintered lead zirconate-titanate
(PZT);
sintering said stacked strips of precursor tape, forming flat PZT
plate members;
separating the sintered plate members from the spacer members;
stacking a plurality of said PZT plate members interspersed with
thermoplastic polymer members;
heating said stack of PZT and polymer members, forming a unitary
multiple-layer laminate of PZT layers spaced from and bonded to one
another by polymer layers;
slicing said laminate along first cutting planes substantially
perpendicular to the planes of said layers of PZT and polymer,
forming substantially flat slab members;
slicing said slab members along second cutting planes perpendicular
to both said first cutting planes and the planes of said layers of
PZT and polymer, forming elongated laminate members comprising a
plurality of layers of PZT spaced by layers of polymer; and
applying conductive electrodes to edges of the PZT layers exposed
on opposite sides of each of the elongated laminate members.
6. The method of claim 5, wherein said polymer material is
polyvinyl butyral or polyvinyl formal.
7. The method of claim 5, comprising the further step of
compressing the stacked polymer and sintered plate members during
said heating step.
8. The method of claim 5, comprising the further step of applying a
vacuum to the stacked polymer and sintered plate members during
said heating step.
9. The method of claim 5, comprising the further step of preparing
said layers of polymer by tape-casting.
10. The method of claim 5, wherein said step of applying conductive
electrodes to exposed edges of the PZT layers on opposite sides of
each elongated laminate member is performed by applying at least
one metallized surface to a first front elongated surface of the
elongated laminate member, and applying a series of individual
electrodes to an opposite rear elongated surface of the elongated
laminate member, and connecting conductors to said metallized
surface and to said series of individual electrodes.
11. The method of claim 10, wherein said step of applying a
metallized surface is performed by electroless plating, by
sputtering, or by vapor deposition.
12. The method of claim 10, wherein said step of applying a series
of individual electrodes is performed by bonding individual traces
on a flex circuit member to exposed edges of the PZT layers using
an anisotropically conductive adhesive material.
13. The method of claim 10, wherein said series of individual
electrodes are each substantially wider than the corresponding
thickness of the PZT members to which the electrodes are applied
such that each electrode is connected to the edges of several PZT
members.
14. The method of claim 10, wherein said step of applying a series
of individual electrodes is performed by bonding individual
conductors formed on a ceramic substrate to the exposed edges of
the PZT layers, using an anisotropically conductive adhesive
material.
15. A method for manufacture of a multiple-element piezoelectric
transducer, comprising the steps of:
preparing a plurality of strips of a green precursor tape including
PZT materials;
stacking said strips interspersed with flat spacer members formed
of a previously-sintered Pb-rich ceramic material;
sintering said strips interspersed with said spacer members, to
form a plurality of flat PZT plate members;
stacking said plurality of flat PZT plate members interspersed with
thermoplastic polymer members;
heating said stack of PZT and polymer members, forming a unitary
multiple-layer laminate of PZT layers spaced from and bonded to one
another by polymer layers;
slicing said laminate along first cutting planes substantially
perpendicular to the planes of said layers of PZT and polymers,
forming substantially flat slab members;
slicing said slab members along second cutting planes perpendicular
to both said first cutting plane and the planes of said layers of
PZT and polymer, forming elongated laminate members comprising a
plurality of layers of PZT spaced by layers of polymer; and
applying conductive electrodes to edges of the PZT layers exposed
on opposite sides of each of the elongated laminate members.
16. The method of claim 15, wherein said polymer material is
polyvinyl butyral or polyvinyl formal.
17. The method of claim 15, comprising the further step of
compressing the stacked polymer and sintered plate members during
said heating step.
18. The method of claim 15, comprising the further step of applying
a vacuum to the stacked polymer and sintered plate members during
said heating step.
19. The method of claim 15, comprising the further step of
preparing said layers of polymer by tape-casting.
20. The method of claim 15, wherein said step of applying
conductive electrodes to exposed edges of the PZT layers on
opposite sides of each of the elongated laminate members is
performed by applying a metallized surface to a first front
elongated surface of each elongated laminate member, and applying a
series of individual electrodes to an opposite rear elongated
surface of the elongated laminate member, and providing connections
to said metallized surface and to said series of individual
electrodes.
21. The method of claim 20, wherein said step of applying a
metallized surface is performed by electroless plating, by
sputtering, or by vapor deposition.
22. The method of claim 20, wherein said step of applying a series
of individual electrodes is performed by bonding individual traces
on a flex circuit member to exposed edges of the PZT layers, using
an anisotropically conductive adhesive material.
23. The method of claim 20, wherein said series of individual
electrodes are each substantially wider than the corresponding
thickness of the PZT members to which the electrodes are bonded,
such that each electrode is bonded to the edges of several PZT
members.
24. The method of claim 20, wherein said step of applying a series
of individual electrodes is performed by bonding individual
conductors formed on a substrate to the exposed edges of the PZT
layers, using an anisotropically conductive adhesive material.
25. A method for manufacture of a plurality of flat plates of a
desired ceramic material, comprising the steps of:
preparing a slurry comprising the desired ceramic material in
powder form in a liquid binder;
forming a green precursor tape by tape-casting said slurry;
allowing said precursor tape to dry;
cutting said precursor tape into strips;
stacking said strips of precursor tape, said strips being
interspersed by and in direct contact with flat spacer members
formed of previously-sintered members of the desired ceramic
material;
sintering said stacked strips of precursor tape and spacer members,
forming flat plate members of said desired ceramic material;
and
removing said spacer members from said flat plate members of said
desired ceramic material.
26. The method of claim 25, wherein said desired ceramic material
is selected from the group including lead zirconate-titanate,
modified lead titanate, lead metaniobate, and barium titanate.
27. The method of claim 25, wherein said sintering step is
performed by disposing said stacked strips of green precursor
material and spacer members in a sintering furnace and operating
said furnace to provide predetermined sintering conditions, and
comprising the further step of providing an oxidizing atmosphere
during said sintering step.
28. The method of claim 25, comprising the further step of
providing a source of one or more of the elements of said desired
ceramic material in said furnace during said sintering step.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to methods of manufacture of
multiple-element piezoelectric transducers, for example, suitable
for ultrasonic imaging applications in medicine and elsewhere.
2. Description of the Prior Art
Multiple-element piezoelectric transducers are known for use in
several applications, in particular for nondestructive imaging of
the interior of structures. For example, endoscopic imaging probes
have been proposed for numerous medical applications. In many such
imaging applications, it is desired to reduce the size of the
individual piezoelectric elements as much as possible, to allow
operation at higher frequencies and thereby provide increased
resolution in the image.
Most present day piezoelectric transducers are fabricated of lead
zirconate-titanate (PZT) ceramic material. The PZT material is
commonly prepared from a PZT powder produced by mixing individual
oxides (PbO, TiO.sub.2, ZrO.sub.2, plus small quantities of
modifiers) and "calcining" the mixture. The calcination step is
accomplished by heating the mixture to a high enough temperature
(e.g., 800.degree.-900.degree. C.) to cause the constituent oxides
to react to form single phase PZT in solid solution. The exact
proportions of the constituent powders are varied in accordance
with the specific properties desired in the PZT material being
prepared. The calcined PZT is then ground to a fine powder, e.g.,
by conventional ball milling techniques. Dense blocks of PZT
ceramic in desired shapes can then be formed by compacting the PZT
powder in a die and then "sintering" the resultant "green" block at
a suitable temperature (typically 1200.degree.-1300.degree. C.).
Hot pressing, in which the PZT powder is subjected to external
pressure during sintering, is also an effective way to produce
blocks of PZT ceramic.
During the sintering process, some of the Pb content tends to be
driven off, particularly from the surface of the pellet, with
adverse effects on the properties of the ceramic; accordingly, it
is known to place a quantity of Pb-containing material, such as
PbZrO.sub.3, in the sintering furnace during the sintering step.
Alternatively, the surface of the sintered block may be removed and
discarded, after which the internal material is sliced into thin
plates which are then lapped flat to form PZT plates for further
processing, such as dicing into small individual elements for
assembly into multiple-element arrays.
U.S. Pat. Nos. 4,514,247 and 4,572,981 to Zola disclose one
proposed alternative in fabrication of multiple-element PZT
transducer arrays. A single plate of PZT material prepared
generally as above is diced into smaller slabs, which are then
stacked interposed with layers of a passive material, such as
glass, paper, phenolic resin, silicone rubber, or another ceramic
material. The PZT slabs may be bonded to the layers of passive
material using epoxy cement. The laminate block thus formed is then
sliced perpendicular to the plane of the layers, resulting in
plates comprising alternating strips of PZT and the passive
material. These plates are then stacked, again interposed with
layers of the passive material, and this assembly is sliced
perpendicular to the strips, to yield one or more planar members in
which PZT elements are surrounded by passive material. Single or
separately-addressable electrodes are then applied to the opposed
planar ends of the planar members.
Production of multiple-element transducer arrays according to the
Zola method requires that the PZT plates be lapped to a desired
thickness. As the lapping process requires a minimum strength, PZT
elements manufactured according to the Zola process have an
undesirably large minimum thickness. Zola's method also requires
that individual members of the passive material be separately
bonded to the PZT elements, which is a time-consuming and complex
process. Zola also fails to teach an efficient method of connection
of individual conductors to the individual PZT elements.
Other methods of fabrication of multiple-element arrays require
multiple fine pitch dicing steps to form finely-divided PZT
elements for subsequent assembly. Assembly of such small elements
is very difficult. Moreover, the PZT material must be relatively
fine grained (grain size .ltoreq.2 microns) to withstand the
abrasive cutting technique employed in the dicing process.
Relatively coarsely-grained PZT material (grain sizes .gtoreq.3-5
microns) exhibits higher piezoelectric sensitivity. Accordingly, At
would be desirable to avoid dicing insofar as possible in the
preparation of multiple-element PZT arrays, to allow use of the
preferable coarsely-grained material.
A technique known as "tape-casting" is commonly employed for
manufacture of "green" precursor tapes formed of materials which
when sintered form a desired ceramic material. In tape-casting, a
slurry is prepared including powders of the materials of the
ceramic of interest, together with organic solvents and binders. A
pool of the slurry, termed the "slip", is poured onto a moving
substrate, between a stationary dam and one or two "doctor blades"
extending parallel to the dam and spaced vertically from the moving
substrate. As the substrate is pulled beneath the doctor blades,
the thickness of the green precursor tape is precisely controlled
by the spacing of the doctor blades from the substrate.
Equivalently, tape casting can be performed by pulling one or a
pair of mobile doctor blades across a stationary substrate. The
green precursor is then dried and sintered to form the ceramic
material. See, e.g., Runk and Andrejco, "A Precision Tape Casting
Machine for Fabricating Thin Ceramic Tapes", Ceramic Bulletin, 54,
No. 2, 199-200 (1975); Mistler, "Tape Casting: The Basic Process
for Meeting the Needs of the Electronics Industry", Ceramic
Bulletin, 69, No. 6, 1022-1026 (1990); Mistler, "Tape Casting", in
Engineered Materials Handbook, 4, 161-165, (1992). See also U.S.
Pat. No. 4,353,958 to Kita et al. Moreover, Wentworth and Taylor,
in "Processing Parameters and Electric Properties of Doctor-Bladed
Ferroelectric Ceramics", Ceramic Bulletin, 46, No. 12, 1186-1193,
(1967), suggest that the properties of Pb(ZrSnTi)O.sub.3
ferroelectric ceramics prepared using doctor-bladed tape-casting
techniques as above may be improved by disposing a source of
Pb-containing material in the sintering furnace during the
sintering step. More specifically, according to this technique, a
green precursor tape is cut into strips and placed between Pb-rich
"setter" members spaced about 2 mils from the surface of the green
precursor strips. During sintering the presence of a Pb-containing
setter increases the local Pb activity, thus minimizing Pb loss
from the green tape and maintaining the proper ceramic
composition.
Other references generally pertinent to the present invention
include U.S. Pat. No. 4,939,826 to Shoup and U.S. Pat. No.
4,564,980 to Diepers, disclosing methods of manufacture of
particular transducers; U.S. Pat. No. 4,977,655 to Martinelli,
disclosing a method for mounting two transducers on a single
catheter, for medical imaging purposes; U.S. Pat. No. 4,518,889 to
'T Hoen teaching an apodized transducer, that is, one having a
particular asymmetric response; and U.S. Pat. No. 4,717,851 to
Fenner et al, disclosing particular acoustic impedance matching
layers for ultrasonic transducers.
OBJECTS OF THE INVENTION
It is therefore an object of the invention to provide an improved
method for the production of flat ceramic members which avoids
extensive sawing or lapping steps practiced in the prior art.
It is therefore a further object of the invention to provide an
improved method for the production of multiple-element
piezoelectric transducers in which the grain size of the
piezoelectric material is not limited by the processing steps
employed.
It is still a further object of the invention to provide a
simplified method for the production of multiple-element
piezoelectric transducers.
It is yet a further object of the invention to provide an improved
method for the production of multiple-element piezoelectric
transducers including a simplified method for connection of
conductors to the piezoelectric elements of the transducer.
It is a further object of the invention to provide an improved
multiple-element piezoelectric transducer.
SUMMARY OF THE INVENTION
The present invention meets the needs of the art and objects of the
invention mentioned above, as well as others apparent from the
following discussion, by provision of an improved method for
fabrication of a multiple-element transducer and the transducer
produced thereby. According to the invention, a green precursor
tape is produced by doctor-blade tape-casting of a slurry
containing PZT powder. The precursor tape is dried. Individual
strips of the green tape are stacked between flat spacers of
previously sintered PZT. The stacked strips are sintered to form
PZT plates. The previously-sintered PZT spacers maintain a high
degree of local Pb activity to minimize Pb loss from the surfaces
of the tape strips during sintering.
The PZT plates are then separated from the previously sintered PZT
spacers, and are stacked interposed by layers of a thermoplastic
polymer. This stack is then heated while under compression, and
while a vacuum is applied, to a temperature above the melting point
of the polymer, forming a laminate block. This block is then sliced
perpendicular to the plane of the layers, forming slabs of
alternate PZT and polymer layers; the slabs are then sliced
perpendicular to the first slicing planes, forming elongate members
of alternating layers of PZT and the polymer material.
A front electrode is then formed on one side of each of the
elongate members, and a series of rear electrodes is applied to the
opposite side of each of the elongate members using an
anisotropically-conductive adhesive. The lateral pitch of the
electrodes may be such that several PZT elements are connected to a
single electrode. One or more layers of a impedance-matching
materials are then provided, together with a backing layer, forming
a complete transducer head assembly.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will be better understood if reference is made to the
accompanying drawings, wherein:
FIG. 1, including FIGS. 1(a)-1(h), comprises a sequence of
perspective schematic views illustrating the principal stages in
fabrication of a multiple-element transducer according to the
invention;
FIG. 2, including FIGS. 2(a)-2(h), comprises a corresponding
sequence of verbal descriptions of the principal stages in
fabrication of a multiple-element transducer according to the
invention;
FIG. 3 is a cross-sectional view of the distal tip of an ultrasonic
probe comprising a multiple-element piezoelectric transducer
according to the invention; and
FIG. 4 is a view corresponding to FIG. 3, illustrating a somewhat
different embodiment of the invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
As indicated above, FIGS. 1 and 2 provide corresponding schematic
illustrations and verbal explanations of the stages in fabrication
of multiple-element piezoelectric transducers according to the
invention. FIGS. 1(a) and 2(a) describe the formation of a green
precursor tape. In the preferred embodiment, the desired
piezoelectric ceramic material to be prepared is lead
zirconate-titanate (PZT). In this case, a PZT powder is prepared by
mixing and calcining PbO, TiO.sub.2, ZrO.sub.2, and small
quantities of modifier oxides in the appropriate proportions. The
properties of the PZT can be varied by suitably varying the ratios
of the constituent oxides. The calcined PZT is then processed to
form a PZT powder, e.g., by ball milling. In accordance with the
teachings of the ceramic tape-casting art discussed above, the PZT
powder is further mixed with known organic solvents, plasticizers,
binders and the like to form a slurry. The slurry is then poured
onto a substrate 14 moving with respect to a pair of double doctor
blades 12 in a generally conventional tape casting system 10, the
slurry thus forming the "slip" of the tape-casting process. The
slip is confined between a dam 18 and the doctor blades 12, so that
as the substrate 14 passes slowly beneath the doctor blades 12, a
green precursor tape 16 is formed, the tape 16 being of very
consistent thickness. The green precursor tape 16 is then dried;
typically air-drying is sufficient.
According to one aspect of the invention, and as depicted in FIGS.
1(b) and 2(b), the dried green precursor tape 16 is then cut into
strips 20, which are stacked, interposed with spacers 22 of
previously sintered PZT material, after which the stack is
sintered, such that the green precursor strips 20 are converted to
sintered ceramic PZT plates. There are several advantages to thus
sintering the green precursor strips 20 while interposed in a stack
with previously sintered PZT spacers 22. As noted, the previously
sintered PZT spacers 22 maintain a high local Pb activity which
minimizes Pb loss from the green precursor strips 20 during
sintering. Preferably, the previously sintered PZT spacers 22 are
lapped flat, form the uppermost and lowermost elements in the
stack, and are in direct contact with the green precursor strips
20, to ensure that the sintered PZT ceramic plates formed upon
sintering the green precursor strips 20 are flat, eliminating any
necessity of lapping the PZT ceramic plates.
It will be appreciated that according to this aspect of the
invention, the green precursor strips are prepared as a thin (e.g.,
100 micron) tape 16, which is subsequently cut into strips and
fired. Since such a thin tape has a relatively high ratio of
surface area to volume, the problem of Pb loss during sintering
will be particularly acute if not avoided. Further, the thin PZT
plates resulting from sintering of such a thin tape are not strong
enough to be lapped, whereby a Pb-depleted surface layer might be
removed. According to the invention, Pb-containing previously
sintered PZT spacers 22 are provided in direct contact with the
green precursor strips 20 during the sintering step. The PZT
spacers 22 separate the green precursor strips 20, minimize Pb
loss, and constrain the PZT plates formed upon sintering of the
strips 20 to remain flat, such that lapping of the PZT plates is
unnecessary. As is well known, the green precursor strips 20 shrink
substantially during sintering, such that the PZT plates formed do
not tend to adhere to the previously sintered PZT spacers 22.
The sintering step may be carried out by placing the stacked green
precursor strips 20 and previously sintered PZT spacers 22 in an
alumina crucible, placing the crucible in a sintering furnace
having an oxidizing atmosphere, and gradually raising the
temperature to on the order of 500.degree. C. to volatilize and
remove the organic binders, plasticizers, and the like used to
provide a workable slurry. The temperature may then be raised to
substantially 1250.degree. C. and maintained for on the order of
0.5 hour, to densify and solidify the green precursor material,
forming solid PZT plates between the previously sintered PZT
spacers 22. It may further be preferable to place a quantity of
PbZrO.sub.3 powder in the sintering crucible, to ensure sufficient
Pb is available during the sintering step. The resulting products
are thin (as little as 15 microns) plates of PZT having desired
relatively coarse grain sizes of 5-10 microns. According to this
aspect of the method of the invention, these products are obtained
without sawing or lapping steps; stated differently, according to
the invention, PZT plates may be produced having piezoelectric
properties which are not compromised by the mechanical steps
necessary for their production.
The same process may be employed for preparing plates of other
ceramic materials, such as modified lead titanates, e.g.,
(Pb,Ca)TiO.sub.3 and (Pb,Sm)TiO.sub.3, lead metaniobate, or barium
titanate, which are used for other purposes. Again, the method of
the invention allows preparation of ceramic members of desired
properties free from constraints imposed by other preparation
methods involving extensive sawing or lapping operations.
FIGS. 1(d)-(h) and 2(d)-(h) illustrate the principal steps
performed in fabrication of multiple-element piezoelectric
transducers using PZT plates produced using the methods described
above. At the step shown by FIGS. 1(d) and 2(d), the sintered PZT
plates 30 are assembled in a stack 34, the PZT plates 30 being
interposed with layers 32 of a thermoplastic polymer, such as
polyvinyl butyral (PVB) or polyvinyl formal (PVF). Manufacture of
the transducer produced according to the method of the invention is
significantly simplified if the number of total PZT elements in a
single row of the complete transducer is equal to the number of PZT
plates 30 in the stack 34. Several hundred alternate layers 32 of
the polymer and plates 30 of PZT material may therefore be provided
in stack 34.
As indicated by FIGS. 1(c) and 2(c), the PVB or PVF material may be
prepared as a sheet 28, using a tape-casting process as discussed
generally above, but this is not a limitation on the invention.
Tape-casting the polymer allows selection from a wide range of
polymer thicknesses, and allows convenient introduction of
plasticizers and fillers to tailor the acoustic impedance and
attenuative properties of the polymer sheets.
The stack 34 of alternating PZT plates 30 and polymer layers 32 is
then placed in a furnace and baked to reach a temperature at least
equal to the melting point of the polymer material (for example,
160.degree. C. for PVB or 210.degree. C. for PVF), such that the
polymer when cooled bonds the PZT plates 30 into a solid block 36.
As indicated, axial compression may be applied during the baking
step, and this step may be carried out under vacuum, to improve the
properties of the block 36. Thus, the polymer both provides a
passive material separating the PZT plates and bonds the PZT plates
into a unitary block. The block 36 having plates 30 of PZT
interspersed with passive polymer layers 32 is referred to in the
art as a "2--2 composite". If prepared as indicated, no voids will
exist between the plates 30 of the PZT ceramic material.
As depicted in FIGS. 1(e) and (f) and 2(e) and (f), the block 36 is
then slit into elongated laminate members 40 comprising alternating
PZT elements 42 and polymer layers 44, in two slitting steps. The
block 36 is first slit along a number of parallel first cutting
planes perpendicular to the planes of the plates of PZT material
and polymer (FIG. 1(e) ), forming slabs 38 comprising alternate
layers of the PZT and polymer materials. The slabs 38 are then slit
along a number of parallel second cutting planes perpendicular to
the first cutting planes and to the planes of the plates of PZT
material and polymer (FIG. 1(f)), forming elongated laminate
members 40, of generally square cross-section, having alternate PZT
elements 42 spaced by layers 44 of polymer extending along their
length.
As illustrated by FIGS. 1(g) and 2(g), front and rear electrodes
are then applied to each elongated laminate member 40 to fabricate
a multiple-element transducer. Several possible embodiments of the
structure of the electrodes are detailed below in connection with
FIGS. 3 and 4. Typically, the front electrode 46 may comprise a
continuous metallized surface applied by electroless plating,
sputtering, or vapor deposition directly to one side of the
elongated laminated member 40. Individual rear electrodes 48 are
connected to the PZT elements 42. The transducer is completed by
impedance matching layers 62 and 64 and a backing layer 60, as
illustrated in FIGS. 1(h) and 2(h).
The rear electrodes 48 may comprise individual traces printed on a
flex circuit element 50 (i.e., individual conductors formed as part
of a printed circuit on a flexible substrate) at the same spacing
as the PZT elements 42 and bonded thereto by a film 56 of a
commercially available anisotropically conductive adhesive
material, e.g., "Z-Axis Adhesive Film" available as Part No. 5303R
from the 3M Corporation. See FIG. 3. This method of connecting the
electrodes 48 to the PZT elements 42 requires that the pitch (i.e.,
the spacing) of the electrodes 48 and PZT elements 42 be equal over
the length of the transducer assembly.
In an alternative embodiment of the invention shown in FIG. 4, the
PZT elements 42 may be formed at substantially finer pitch than the
rear electrodes 58, such that each electrode 58 is connected (using
the same conductive adhesive) to several, typically two or three,
of the PZT elements 42. Connecting each electrode 58 to several of
the PZT elements 42 renders the relative pitch thereof less
critical. FIG. 4 also illustrates an alternative method of forming
the electrodes 58, namely by printing directly onto a ceramic
substrate 54. The electrodes 58 and the conductors 70 to which the
electrodes are connected might also be printed directly onto a
backing layer 60 typically applied to protect the rear surface of
the laminate member 40. Backing layer 60 is typically also provided
in the embodiment of FIG. 3, as shown.
As further illustrated in FIGS. 3 and 4, a multiple-element
transducer 66 according to the invention is completed by one or
more layers 62, 64 of acoustic impedance matching materials. Layers
62, 64 are provided in order to match the very high acoustic
impedance of the hard ceramic elements 42 to the material with
which the transducer 66 is intended to be used. Where the
transducer 66 is to be used for medical imaging purposes, such that
the transducer 66 will function to transmit ultrasonic energy into
body tissues and detect reflection of energy from internal
structures and the like, suitable impedance matching layers 62 and
64 may be formed of glass and epoxy, respectively, as generally
known in the art. As noted above, a backing layer 60 is commonly
provided as well; a suitable material is tungsten-loaded epoxy,
again as generally known in the art.
A complete transducer 66 may include a single elongate laminate
assembly as described above, for emitting energy into a structure
to be examined and detecting energy reflected therefrom, may
comprise several such assemblies arranged in line with one another,
providing a longer one-dimensional array of transducer elements, or
may comprise a number of such assemblies in a two-dimensional
array. The transducer 66 may be conveniently mounted at the distal
tip of a probe 68, for being maneuvered into contact with tissues
or other structures to be imaged or otherwise examined using energy
emitted by the transducer 66. Conductors 70 connected to the
electrodes 46, 48 pass through a lumen extending along the axis of
probe 68 and are connected to excitation and analysis circuitry,
for supplying energy transmitted into the structure to be examined
and analysis of the reflected energy, all generally as known in the
art. Typically pulses of high-frequency energy will be applied
between the front electrode 46 and selected ones of the rear
electrodes 48, to cause the corresponding PZT elements 42 to
transmit bursts of ultrasonic energy into the structure to be
examined; electrical signals emitted by each of the PZT elements 42
responsive to detection of reflected energy are then transmitted by
conductors 70 to data analysis and imaging equipment, again
generally as known to the art.
While a preferred embodiment of the invention has been disclosed in
detail, this exemplary disclosure should not be considered to limit
the invention, which is limited only by the following claims.
* * * * *